here \(\left|\eta \right\rangle \)  is the entangled states in two-mode
Fock space [1]}, [2]}, [3]}, [4]} named after Einstein-Podolsky-Rosen (EPR)'s [5]} concept of quantum entanglement,
\(\left|\eta \right\rangle =\exp \lbrace -\frac{1}{2}\left|\eta \right|^{2}+\eta a_{1}^{\dagger }-\eta ^{\ast }a_{2}^{\dagger }+a_{1}^{\dagger }a_{2}^{\dagger }\rbrace \left|00\right\rangle . \) 
